1、沈阳建筑大学毕业论文外文及翻译原 文 题 目 Android Application Fundamentals 学 院 专 业 班 级 信息与控制工程学院 计算机 08-1 学 生 姓 名 XXX 性别 X 指 导 教 师 XXX 职称 XXX 年 月 日沈阳建筑大学毕业论文共 21 页 第 1 页外文及翻译英语原文Android Application FundamentalsAndroid applications are written in the Java programming language. The Android SDK tools compile the codealon
2、g with any data and resource filesinto an Android package, an archive file with an .apk suffix. All the code in a single .apk file is considered to be one application and is the file that Android-powered devices use to install the application.Once installed on a device, each Android application live
3、s in its own security sandbox: The Android operating system is a multi-user Linux system in which each application is a different user. By default, the system assigns each application a unique Linux user ID (the ID is used only by the system and is unknown to the application). The system sets permis
4、sions for all the files in an application so that only the user ID assigned to that application can access them. Each process has its own virtual machine (VM), so an applications code runs in isolation from other applications. By default, every application runs in its own Linux process. Android star
5、ts the process when any of the applications components need to be executed, then shuts down the process when its no longer needed or when the system must recover memory for other applications.In this way, the Android system implements the principle of least privilege. That is, each application, by d
6、efault, has access only to the components that it requires to do its work and no more. This creates a very secure environment in which an application cannot access parts of the system for which it is not given permission.However, there are ways for an application to share data with other application
7、s and for an application to access system services: Its possible to arrange for two applications to share the same Linux user ID, in which case they are able to access each others files. To conserve system resources, 沈阳建筑大学毕业论文共 21 页 第 2 页applications with the same user ID can also arrange to run in
8、 the same Linux process and share the same VM (the applications must also be signed with the same certificate). An application can request permission to access device data such as the users contacts, SMS messages, the mountable storage (SD card), camera, Bluetooth, and more. All application permissi
9、ons must be granted by the user at install time.That covers the basics regarding how an Android application exists within the system. The rest of this document introduces you to: The core framework components that define your application. The manifest file in which you declare components and require
10、d device features for your application. Resources that are separate from the application code and allow your application to gracefully optimize its behavior for a variety of device configurations. Application ComponentsApplication components are the essential building blocks of an Android applicatio
11、n. Each component is a different point through which the system can enter your application. Not all components are actual entry points for the user and some depend on each other, but each one exists as its own entity and plays a specific roleeach one is a unique building block that helps define your
12、 applications overall behavior.There are four different types of application components. Each type serves a distinct purpose and has a distinct lifecycle that defines how the component is created and destroyed.Here are the four types of application components:ActivitiesAn activity represents a singl
13、e screen with a user interface. For example, an email application might have one activity that shows a list of new emails, another activity to compose an email, and another activity for reading emails. Although the activities work together to form a cohesive user experience in the email application,
14、 each one is independent of the others. As such, a different application can start any one of these activities (if the email application allows it). For example, a camera application can start the activity in the email application that composes new mail, in order for the user 沈阳建筑大学毕业论文共 21 页 第 3 页t
15、o share a picture.An activity is implemented as a subclass of Activity and you can learn more about it in the Activities developer guide.ServicesA service is a component that runs in the background to perform long-running operations or to perform work for remote processes. A service does not provide
16、 a user interface. For example, a service might play music in the background while the user is in a different application, or it might fetch data over the network without blocking user interaction with an activity. Another component, such as an activity, can start the service and let it run or bind
17、to it in order to interact with it.A service is implemented as a subclass of Service and you can learn more about it in the Services developer guide.Content providersA content provider manages a shared set of application data. You can store the data in the file system, an SQLite database, on the web
18、, or any other persistent storage location your application can access. Through the content provider, other applications can query or even modify the data (if the content provider allows it). For example, the Android system provides a content provider that manages the users contact information. As s
19、uch, any application with the proper permissions can query part of the content provider (such as ContactsContract.Data) to read and write information about a particular person.Content providers are also useful for reading and writing data that is private to your application and not shared. For examp
20、le, the Note Pad sample application uses a content provider to save notes.A content provider is implemented as a subclass of ContentProvider and must implement a standard set of APIs that enable other applications to perform transactions. For more information, see the Content Providers developer gui
21、de.Broadcast receiversA broadcast receiver is a component that responds to system-wide broadcast 沈阳建筑大学毕业论文共 21 页 第 4 页announcements. Many broadcasts originate from the systemfor example, a broadcast announcing that the screen has turned off, the battery is low, or a picture was captured. Applicatio
22、ns can also initiate broadcastsfor example, to let other applications know that some data has been downloaded to the device and is available for them to use. Although broadcast receivers dont display a user interface, they may create a status bar notification to alert the user when a broadcast event
23、 occurs. More commonly, though, a broadcast receiver is just a “gateway“ to other components and is intended to do a very minimal amount of work. For instance, it might initiate a service to perform some work based on the event.A broadcast receiver is implemented as a subclass of BroadcastReceiver a
24、nd each broadcast is delivered as an Intent object. For more information, see theBroadcastReceiver class.A unique aspect of the Android system design is that any application can start another applications component. For example, if you want the user to capture a photo with the device camera, theres
25、probably another application that does that and your application can use it, instead of developing an activity to capture a photo yourself. You dont need to incorporate or even link to the code from the camera application. Instead, you can simply start the activity in the camera application that cap
26、tures a photo. When complete, the photo is even returned to your application so you can use it. To the user, it seems as if the camera is actually a part of your application.When the system starts a component, it starts the process for that application (if its not already running) and instantiates t
27、he classes needed for the component. For example, if your application starts the activity in the camera application that captures a photo, that activity runs in the process that belongs to the camera application, not in your applications process. Therefore, unlike applications on most other systems,
28、 Android applications dont have a single entry point (theres no main() function, for example).Because the system runs each application in a separate process with file permissions that restrict access to other applications, your application cannot directly activate a component from another applicatio
29、n. The Android system, however, can. So, to activate a component in another application, you must deliver a message to the system that specifies your intent to start a particular component. The system then activates the component for you.沈阳建筑大学毕业论文共 21 页 第 5 页Activating ComponentsThree of the four c
30、omponent typesactivities, services, and broadcast receiversare activated by an asynchronous message called an intent. Intents bind individual components to each other at runtime (you can think of them as the messengers that request an action from other components), whether the component belongs to y
31、our application or another.An intent is created with an Intent object, which defines a message to activate either a specific component or a specific type of componentan intent can be either explicit or implicit, respectively.For activities and services, an intent defines the action to perform (for e
32、xample, to “view“ or “send“ something) and may specify the URI of the data to act on (among other things that the component being started might need to know). For example, an intent might convey a request for an activity to show an image or to open a web page. In some cases, you can start an activit
33、y to receive a result, in which case, the activity also returns the result in an Intent (for example, you can issue an intent to let the user pick a personal contact and have it returned to youthe return intent includes a URI pointing to the chosen contact).For broadcast receivers, the intent simply
34、 defines the announcement being broadcast (for example, a broadcast to indicate the device battery is low includes only a known action string that indicates “battery is low“).The other component type, content provider, is not activated by intents. Rather, it is activated when targeted by a request f
35、rom a ContentResolver. The content resolver handles all direct transactions with the content provider so that the component thats performing transactions with the provider doesnt need to and instead calls methods on the ContentResolver object. This leaves a layer of abstraction between the content p
36、rovider and the component requesting information (for security).There are separate methods for activating each type of component: You can start an activity (or give it something new to do) by passing an Intent to startActivity() or startActivityForResult() (when you want the activity to return a res
37、ult).沈阳建筑大学毕业论文共 21 页 第 6 页 You can start a service (or give new instructions to an ongoing service) by passing an Intent to startService(). Or you can bind to the service by passing an Intent tobindService(). You can initiate a broadcast by passing an Intent to methods like sendBroadcast(), sendOrd
38、eredBroadcast(), or sendStickyBroadcast(). You can perform a query to a content provider by calling query() on a ContentResolver.For more information about using intents, see the Intents and Intent Filters document. More information about activating specific components is also provided in the follow
39、ing documents: Activities, Services, BroadcastReceiver and Content Providers. Declaring componentsThe primary task of the manifest is to inform the system about the applications components. For example, a manifest file can declare an activity as follows:.In the element, the android:icon attribute po
40、ints to resources for an icon that identifies the application.In the element, the android:name attribute specifies the fully qualified class name of the Activity subclass and the android:label attributes specifies a string to use as the user-visible label for the activity.You must declare all applic
41、ation components this way:沈阳建筑大学毕业论文共 21 页 第 7 页 elements for activities elements for services elements for broadcast receivers elements for content providersActivities, services, and content providers that you include in your source but do not declare in the manifest are not visible to the system a
42、nd, consequently, can never run. However, broadcast receivers can be either declared in the manifest or created dynamically in code (as BroadcastReceiver objects) and registered with the system by calling registerReceiver().Declaring component capabilitiesAs discussed above, in Activating Components
43、, you can use an Intent to start activities, services, and broadcast receivers. You can do so by explicitly naming the target component (using the component class name) in the intent. However, the real power of intents lies in the concept of intent actions. With intent actions, you simply describe t
44、he type of action you want to perform (and optionally, the data upon which youd like to perform the action) and allow the system to find a component on the device that can perform the action and start it. If there are multiple components that can perform the action described by the intent, then the
45、user selects which one to use.The way the system identifies the components that can respond to an intent is by comparing the intent received to the intent filters provided in the manifest file of other applications on the device.When you declare a component in your applications manifest, you can opt
46、ionally include intent filters that declare the capabilities of the component so it can respond to intents from other applications. You can declare an intent filter for your component by adding an element as a child of the components declaration element.For example, an email application with an acti
47、vity for composing a new email might declare an intent filter in its manifest entry to respond to “send“ intents (in order to send email). An activity in your application can then create an intent with the “send” action (ACTION_SEND), which the system matches to the email applications “send” activit
48、y and launches it when you invoke the intent with startActivity().沈阳建筑大学毕业论文共 21 页 第 8 页For more about creating intent filters, see the Intents and Intent Filters document.Declaring application requirementsThere are a variety of devices powered by Android and not all of them provide the same feature
49、s and capabilities. In order to prevent your application from being installed on devices that lack features needed by your application, its important that you clearly define a profile for the types of devices your application supports by declaring device and software requirements in your manifest file. Most of these declarations are informational only and the system does not read them, but external services such as Google Play do read them in order to provide filtering for users when they search for a
